Digital to analog converters are devices that accept a digital code and convert it to an analog quantity, typically an analog output voltage but some converters are arranged to provide analog currents or packets of charge. It is further known by the person skilled in the art that given a known current one can produce a known voltage, and that given a known voltage one can produce a known current. In, for example, voltage based digital to analog converters it is generally desirable to ensure that the output voltage of the digital to analog converter is not effected by the electrical characteristics, such as impedance, of a circuit being driven by the digital to analog converter. It is therefore known to provide buffer amplifiers in association with digital to analog converters where the buffer amplifier is responsible for driving the load. Desirably the buffer presents a substantially constant load to the digital to analog converter. More generally buffer amplifiers can be provided whenever it is desired to amplify a signal or to provide an impedance transformation for example from a high impedance to a low impedance, or to separate a circuit from the effects of a load that may be coupled to the circuit or the voltages at which the load operates. The provision of a buffer amplifier may help the system designer better achieve the competing requirements of providing a low noise, fast settling, digital to analog converter while seeking to keep power consumption low.